The present invention relates generally to radio transmitters, in particular mobile telephones, and more particularly to antennas for inclusion in such transmitters and including a conductive layer.
This kind of antenna includes a patch which is typically obtained by etching a metal layer. It is then called a microstrip patch antenna.
The microstrip technique is a planar technique that is used to make transmission lines for transmitting guided waves, possibly carrying signals, and antennas coupling such lines and radiated waves. It uses conductive patches and/or strips formed on the top surface of a thin dielectric substrate. A conductive layer extends over the bottom surface of the substrate and constitutes an earth layer of the line or the antenna. The patch is typically wider than the strip and its shape and dimensions are important characteristics of the antenna. The substrate is typically a plane rectangular sheet of constant thickness and the patch is also typically rectangular. This is not obligatory, however. In particular, it is known in the art that varying the thickness of the substrate can enlarge the bandwidth of this kind of antenna and that the patch can be various shapes, for example circular. The electric field lines extend between the strip or the patch and the earth layer through the substrate. A transmission line operating in the above manner is referred to hereinafter as a microstrip line.
The above technology differs from coplanar technologies that also use conductive elements on a thin substrate, and in particular the transmission line technology in which the electric field is established on the top surface of the substrate and symmetrically between a central conductive strip and two conductive lands on respective opposite sides of the strip, from which they are separated by respective slots. A transmission line operating in this manner is referred to hereinafter as a coplanar line. In an antenna using this technology, a patch is surrounded by a continuous conductive land from which it is separated by a slot.
In another coplanar technology, a transmission line is formed by a slot in a conductive layer and the electric field of the transmitted wave is established in the plane of that layer between the two edges of the slot.
Antennas using the above technologies typically (although not necessarily) constitute resonant structures in which standing waves are established that provide coupling with waves radiated in space.
Various resonant structures of the above kind can be made, for example using the microstrip technology, and each such structure can support one or more resonant modes, referred to for brevity hereinafter as xe2x80x9cresonancesxe2x80x9d. Broadly speaking, each resonance can be defined as a standing wave formed by the superposition of two travelling waves propagating along the same path in opposite directions and resulting from alternating reflection at the two ends of the path of the same travelling wave, which is an electromagnetic wave propagating along that path in a line consisting of the earth layer, the substrate and the patch, for example. The path is imposed by the components of the antenna. It can be rectilinear or curved. It is referred to hereinafter as the xe2x80x9cresonance pathxe2x80x9d. The resonant frequency is inversely proportional to the time taken by the travelling wave referred to above to travel this path.
In a first type of resonance, which is referred to as xe2x80x9chalf-wavexe2x80x9d resonance, the length of the resonance path is typically substantially equal to one half-wavelength, i.e. to half the wavelength of the travelling wave referred to above. The antenna is then referred to as a xe2x80x9cxe2x80x9chalf-wavexe2x80x9d antenna. This type of resonance can be generally defined by the presence of an electrical current node at each of the two ends of a path of this kind, whose length can therefore also be equal to said half-wavelength multiplied by an integer other than one, and typically an odd number. The two ends of the path are located in regions in which the amplitude of the electrical field that is applied via the substrate, for example, is at a maximum; coupling with the radiated waves occurs at one or both ends of the path.
A second type of resonance that can be obtained using the same technology is referred to as xe2x80x9cquarter-wavexe2x80x9d resonance and differs from half-wave resonance firstly in that the resonance path typically has a length equal to one quarter-wavelength, i.e. one quarter of the wavelength defined above. To this end the resonant structure must include a short circuit at one end of the path, the term xe2x80x9cshort circuitxe2x80x9d referring to a connection between the earth layer and the patch. The short circuit must have a sufficiently low impedance to impose the resonance. This type of resonance can be generally defined by the presence of an electric field node fixed by this kind of short circuit in the vicinity of an edge of the patch and by an electrical current node at the other end of the resonance path. The length of the resonance path can therefore be equal to an integer number of half-wavelengths added to said quarter-wavelength. Coupling with waves radiated in space occurs at an edge of the patch, in a region where the amplitude of the electric field is sufficiently high.
Other types of resonance can be established, each characterized by a distribution of the electric and magnetic fields that oscillate in an area of space including the antenna and its immediate vicinity. They depend in particular on the configuration of the patches, which can incorporate slots, possibly radiating slots. In the case of microstrip antennas, the resonances are also conditioned by the presence and location of any short circuits and by the electrical models representing the short circuits if the latter are imperfect, i.e. if they cannot be considered even approximately equivalent to perfect short circuits of zero impedance.
The presence of an imperfect short circuit in an antenna can cause a resonance having what is referred to as a virtual node, which occurs when the following conditions are met (in what follows, the antenna discussed above is referred to as the xe2x80x9cfirst antennaxe2x80x9d):
The distribution of fields in the first antenna is substantially identical to a distribution that can be induced in an identical area of the patch of a second antenna.
The second antenna is identical to the first antenna, within the limits of the area, except that the second antenna has no short circuit.
The patch of the second antenna extends not only over the area already mentioned, which then constitutes a principal area of the second antenna, but also over an additional area.
Finally, the field distribution in question in the principal area of the second antenna is accompanied by an electric or magnetic field node in the additional area.
In order to describe the resonance occurring in the first antenna, the node occurring in the second antenna can be considered to constitute a node for the resonance of the first antenna also. For an antenna such as the first antenna this kind of node is referred to hereinafter as a xe2x80x9cvirtualxe2x80x9d node, because it is located in an area outside the patch of the antenna and in which there is therefore no electric or magnetic field enabling the presence of the node to be determined directly.
Although these xe2x80x9cvirtual nodesxe2x80x9d are not conventionally taken into account in those terms when describing resonances, they are implied by the distinction that is sometimes made between the physical or geometrical length of the patch and its so-called xe2x80x9celectricalxe2x80x9d length. In the case of the two antennas considered above, the physical or geometrical length of the patch of the first antenna would be that of the patch, but the electrical length of the patch would in fact be the physical or geometrical length of the patch of the second antenna.
An antenna is typically coupled to a signal processor such as a transmitter by a connection system including a connection line which is external to the antenna and terminates in a coupling system integrated into the antenna for coupling the line to one or more resonances that can be established in one or more resonant structures of the antenna. The resonances also depend on the nature and location of the connection system, which enables the antenna to be used at each resonant frequency. In the case of a transmit antenna, the connection system is often referred to as a feed line of the antenna.
The present invention concerns various types of device, such as mobile telephones, base stations for mobile telephones, vehicles, aircraft and missiles. In the case of a mobile telephone, the continuous nature of the bottom earth layer of a microstrip antenna facilitates limiting the amount of radiated power that is intercepted by the user""s body. In the case of vehicles, and above all in the case of aircraft or missiles, whose exterior surface is metallic and has a curved profile to produce a low aerodynamic drag, the antenna can be conformed to that profile so that no unwanted additional aerodynamic drag is produced.
The invention relates more particularly to the situation in which an antenna with a conductive layer must have the following qualities:
it must be able to transmit and/or receive radiated waves efficiently on two separate frequencies with a large difference between them,
it must be possible to connect it to a signal processor by means of a single connecting line for all operating frequencies of a transmission device without causing an unwanted spurious standing wave ratio on the line, and
it must achieve this without using a frequency multiplexer or demultiplexer.
Many prior art microstrip antennas with the above three qualities have been made or proposed. They differ in terms of how they establish and couple a plurality of different resonant frequencies. Several such antennas are examined below.
A first such prior art antenna is described in U.S. Pat. No. 4,692,769 (Gegan). In a first embodiment, the patch of the antenna takes the form of a circular disk 10 enabling the antenna to exhibit two half-wave resonances whose paths are respectively established along a diameter AA of the disk and along a circular arc slot 24 inscribed in the disk. The coupling system takes the form of a line 16 constituting a quarter-wave transformer and connected at an interior point to the area of the patch so that the real part of the input impedance of the antenna has substantially the same value for the two resonances. Impedance-matching slots 26 and 28 are concentrically inscribed in the disk 10 so that the imaginary part of the input impedance also has substantially the same value for the two resonances. The line 16 is a microstrip line. In other words, it is not made using the coplanar line technology described hereinabove. However, the document also states that the line is coplanar, but this merely indicates that the strip of the microstrip line is in the same plane as the patch 10. Two slots are formed in the conductive layer of the patch, one on each side of the strip, to enable a terminal segment of the line to penetrate into the area of the patch without causing unwanted contact of the strip with the patch in that segment. One of the two slots is continued by an extension that constitutes the impedance-matching slot 28, and so the line 16 appears to exhibit asymmetry at the end inside the patch 10. Despite the apparent continuity and asymmetry, the skilled person will realize that in practice no wave propagates over the length of the impedance-matching slot 28.
A second prior art antenna is described in U.S. Pat. No. 4,766,440 (Gegan). The general shape of the patch 10 of this antenna is rectangular, enabling the antenna to exhibit two half-wave resonances whose paths are established along a length and a width of the patch. It also features a U-shaped curved slot which is entirely inside the patch. The slot is a radiating slot and establishes an additional resonance mode along another path. By appropriately choosing its shape and dimensions, the frequencies of the resonance modes can have any required value, which opens up the possibility of transmitting a circular polarization wave by associating two modes having the same frequency and crossed linear polarizations. The coupling system takes the form of a microstrip line, but it is also stated to be coplanar, as in the Gegan U.S. Pat. No. 4,692,769 previously cited. The coupling system is provided with an impedance conversion system for matching it to the different input impedances of the line at the different resonant frequencies used as working frequencies.
A third prior art antenna differs from the previous two antennas in that it uses a single resonance path. It is described in U.S. Pat. No. 4,771,291 (LO et al.). Its patch includes point short circuits and slots extending along straight line segments inside the patch. The slots and short circuits reduce the difference between two frequencies corresponding to two resonances having said path in common but respective different modes, designated (0,1) and (0,3), i.e. the common path is occupied by one half-wave or by three half-waves, according to the mode concerned. The ratio between the two frequencies can therefore be reduced from 3 to 1.8. The point short circuits consist of conductors passing through the substrate. The coupling system is a coaxial line whose central conductor passes through the substrate of the antenna in order to be connected to its patch and whose earth conductor is connected to the earth layer of the antenna.
The above antenna has the particular disadvantage that its fabrication is complicated by the incorporation of point short circuits.
A fourth prior art two-frequency antenna differs from the previous three antennas in that it uses a quarter-wave resonance. It is described in IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM DIGEST, NEWPORT BEACH, Jun. 18-23, 1995, pages 2124-2127 Boag et al. xe2x80x9cDual Band Cavity-Backed Quarter-wave Patch Antennaxe2x80x9d. A first resonant frequency is defined by the dimensions and characteristics of the substrate and the patch of the antenna. A resonance of substantially the same type is obtained on the same resonance path at a second frequency by using a matching system.
The coupling system appears to be of the coaxial line type, the matching system being placed at the end of the line, whose axial conductor is extended through the substrate of the antenna and connected to its patch.
Other prior antennas include three conductive layers, namely two superposed patches on top of a common earth layer. They then have the particular drawback that the cumulative thickness of the dielectric substrates between the layers makes the total thickness of the antenna excessive.
As a general rule, the above prior art antennas have the drawback that it is difficult, and therefore costly, to obtain the required resonant frequencies at the same time as good coupling of each resonance to a signal processor.
The objects of the present invention include:
providing a simple way of producing a two-frequency antenna with a coupling system whose impedance is easy to match at each of two resonant frequencies, and
limiting the dimensions of the antenna.
With the above objects in view, the invention provides an antenna with a conductive layer and a coupling system which includes a coplanar line formed by two primary coupling slots in a conductive layer of said antenna. According to the invention, said coupling system further includes a slotted line formed by a slot connected to one of said two primary coupling slots and constituting a secondary coupling slot.
The antenna preferably includes a patch and an earth layer co-operating with said patch in the manner of the microstrip technology, and said coupling slots are formed in said patch. However, another possible arrangement is for a coupling system consisting of such slots to be formed in the earth layer of this kind of antenna.
Said patch preferably includes a separator system including at least one separator slot and defining two areas in said patch respectively constituting:
a primary resonance area including said coplanar line, and
a secondary resonance area including said slotted line.